The yeast protein Nce103p encoded by the gene NCE103 (YNL036w) was described by Cleves et al. (1996) as a substrate of the non-classical export pathway which acts independently of the classical pathway through the ER and the Golgi compartments. However, the predicted amino acid sequence of Nce103p shows high levels of identities to carbonic anhydrases of pro- and eukaryotes. A nce103-Delta deletion strain did not grow on a rich peptone-yeast extract-glucose medium under normal aerobic conditions at pH values of 3.0-8.0, but grew like wild-type in an oxygen-free nitrogen or oxygen-reduced atmosphere over this pH range, and was more sensitive to H(2)O(2) than wild-type. No carbonic anhydrase activity could be detected in crude extracts prepared from wild-type, nce103-Delta mutants or in strains transformed with a multicopy plasmid carrying the NCE103 gene. Expression of the Medicago sativa carbonic anhydrase gene (Coba de la Peña et al., 1997), in a yeast expression cassette on a multicopy plasmid, complemented the growth defects caused by the nce103-Delta deletion and carbonic anhydrase activity could be readily detected in the crude extract. The ability of the nce103-Delta deletion strain to grow like wild-type under anaerobic conditions suggests that the protein encoded by NCE103 is required for protection against certain products of an oxidative metabolism and can be replaced in this function by the Medicago sativa carbonic anhydrase. A NCE103 promoter-LacZ fusion in a wild-type background showed that NCE103 is poorly transcribed under aerobic conditions and at an undetectable level under anaerobic conditions.
Copyright 1999 John Wiley & Sons, Ltd.